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A packaging of the IntelliJ Community Edition python-community library. This is release number 1 of trunk branch 142.
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/*
 * Copyright 2000-2014 JetBrains s.r.o.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 * http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */
package com.jetbrains.python;

import com.intellij.lang.ASTNode;
import com.intellij.lang.LanguageUtil;
import com.intellij.lang.ParserDefinition;
import com.intellij.lang.TokenSeparatorGenerator;
import com.intellij.lexer.Lexer;
import com.intellij.openapi.util.text.StringUtil;
import com.intellij.psi.PsiElement;
import com.intellij.psi.PsiManager;
import com.intellij.psi.TokenType;
import com.intellij.psi.impl.source.tree.Factory;
import com.intellij.psi.util.PsiTreeUtil;
import com.jetbrains.python.lexer.PythonIndentingLexer;
import com.jetbrains.python.psi.PyFunction;
import com.jetbrains.python.psi.PyStatement;
import com.jetbrains.python.psi.impl.PyPsiUtils;

/**
 * @author yole
 */
public class PyTokenSeparatorGenerator implements TokenSeparatorGenerator {
  public ASTNode generateWhitespaceBetweenTokens(ASTNode left, ASTNode right) {
    PsiManager manager = right.getTreeParent().getPsi().getManager();
    if (left.getElementType() == PyTokenTypes.END_OF_LINE_COMMENT) {
      return createLineBreak(manager);
    }

    if (left.getPsi().isValid() && right.getPsi().isValid()) {
      final PsiElement commonParent = PsiTreeUtil.findCommonParent(left.getPsi(), right.getPsi());
      if (commonParent == null) return null;
      final PsiElement leftPrevAncestor = PsiTreeUtil.findPrevParent(commonParent, left.getPsi());
      final PsiElement rightPrevAncestor = PsiTreeUtil.findPrevParent(commonParent, right.getPsi());

      if (isStatementOrFunction(leftPrevAncestor) && isStatementOrFunction(rightPrevAncestor)) {
        int leftIndent = PyPsiUtils.getElementIndentation(leftPrevAncestor);
        int rightIndent = PyPsiUtils.getElementIndentation(rightPrevAncestor);
        int maxIndent = Math.max(leftIndent, rightIndent);
        return createWhitespace(manager, "\n" + StringUtil.repeatSymbol(' ', maxIndent));
      }
    }

    if (right.getElementType() == PyTokenTypes.DEF_KEYWORD || right.getElementType() == PyTokenTypes.CLASS_KEYWORD) {
      return createLineBreak(manager);
    }
    if (left.getElementType() == TokenType.WHITE_SPACE || right.getElementType() == TokenType.WHITE_SPACE) {
      return null;
    }
    final PyStatement leftStatement = PsiTreeUtil.getParentOfType(left.getPsi(), PyStatement.class);
    if (leftStatement != null && !PsiTreeUtil.isAncestor(leftStatement, right.getPsi(), false)) {
      return createLineBreak(manager);
    }
    final Lexer lexer = new PythonIndentingLexer();
    if (LanguageUtil.canStickTokensTogetherByLexer(left, right, lexer) == ParserDefinition.SpaceRequirements.MUST) {
      return createSpace(manager);
    }
    return null;
  }

  private static boolean isStatementOrFunction(PsiElement element) {
    return element instanceof PyFunction || element instanceof PyStatement;
  }

  private static ASTNode createSpace(PsiManager manager) {
    return createWhitespace(manager, " ");
  }

  private static ASTNode createLineBreak(PsiManager manager) {
    return createWhitespace(manager, "\n");
  }

  private static ASTNode createWhitespace(PsiManager manager, final String text) {
    return Factory.createSingleLeafElement(TokenType.WHITE_SPACE, text, 0, text.length(), null, manager);
  }
}